As an inkjet head forming a recording head of an inkjet recording device used as an image recording device or an image forming device, such as a printer, a facsimile, and a copying machine, a head using an electrostatic actuator as disclosed in Japanese Laid-Open Patent Application No. 2001-260346 is well known.
This electrostatic inkjet head includes electrostatic actuators in each of which a diaphragm used also as or including a first electrode forming a wall surface of a discharge room communicating with a nozzle and a second electrode (an individual electrode) are opposed to each other with a predetermined air gap therebetween. A driving waveform is applied between the first electrode and the second electrode of this electrostatic actuator so as to deform the diaphragm of each actuator by utilizing an electrostatic attraction. By a mechanical force upon the deformation, or by a mechanical restitution produced in the diaphragm upon turning off the electrostatic attraction, an ink in the discharge room is discharged from the nozzle.
In a drive control device for a head using such electrostatic actuators, first electrodes of the electrostatic actuators are combined electrically to form a common electrode, and the first electrodes forming the common electrode are set to 0V, and upon discharging an ink drop, a pulse-form potential of +V is selectively applied to individual electrodes (second electrodes).
Besides, as a drive control device for driving an electrostatic actuator, PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998 “A MEMS-Based Projection Display” describes an example in which a nonzero potential is applied to both electrodes composing an actuator of an optical mirror. This drive control device applies a bias potential to a reflector plate, and applies an address potential to an electrode determining a direction of the reflector plate. Upon each control, a potential of 24V to −26V is applied to the reflector plate, and a potential of 0V or 5V is applied to the address electrode. This manner of applying the voltages is devised for maximizing a function of the optical mirror, thereby enabling the reflector plate to surely swing at +10 degrees or −10 degrees according to a control signal, with a remarkably high reliability.
By the way, an inkjet recording device, such as an inkjet printer, is required to have a high total performance, such as an output speed (a recording speed) and an image quality. To fulfill these requirements, a degree of nozzle concentration at a head is raised so as to increase a number of nozzles.
At this point, in view of a relation between an improvement of the degree of nozzle concentration and a head structure, generally, unlike a thermal head discharging an ink from a nozzle by using a pressure of cavities generated by causing the ink to undergo a film boiling by using a heating resistor, a piezoelectric or electrostatic head which includes a diaphragm having a low rigidity, and discharges an ink by varying this diaphragm, has a difficulty in raising the degree of concentration.
In order to raise the degree of concentration in an electrostatic head, a shorter-side width (a width in a direction in which nozzles are arranged) of a diaphragm has to be shortened, whereas a volume of discharged an ink drop has to be secured to a certain degree. Therefore, in order to shorten the shorter-side width of the diaphragm, a displacement of the diaphragm needs to be enlarged. In this case, from a simple viewpoint, thinning a thickness of the diaphragm can enlarge the displacement even though the shorter-side width is short; however, from a viewpoint of discharging a drop, the diaphragm needs to have a certain degree of rigidity, thereby limiting a range in which the diaphragm can be thinned.
That is, an electrostatic attraction generated in an electrostatic actuator can be represented by the following expression (1), where V is a driving voltage, g is a gap length (a distance between an individual electrode and a common electrode), and δ is a displacement of a diaphragm.F=(ε0/2)·V2/(g−δ)2  <Expression 1>
As mentioned above, when the degree of nozzle concentration is raised, the gap length g should be increased. According to the expression (1), when the gap length g becomes large, the driving voltage V also needs to be raised in order to obtain the electrostatic attraction of the same magnitude. Further, as the gap length g becomes larger, the diaphragm displacement δ has a smaller variation range in relation to a variation range of the driving voltage V; therefore, even an slight enlargement of the gap length g calls for a large increase in the driving voltage V. In other words, when the degree of concentration is raised while maintaining a capability of discharging drops, the driving voltage of an actuator tends to be made higher.
Such an increase in the driving voltage means not only an increase in power consumption but also an increase in a withstand pressure of a transistor composing a drive control device (a driver) controlling the actuator. In general, as a size of a transistor becomes larger, a withstand pressure of the transistor becomes higher, although depending also on a thickness of an oxide film of the transistor. Besides, as the withstand pressure becomes higher, a manufacturing process also becomes more costly. As a result, the increase in the driving voltage leads to the cost of the drive control device becoming higher. In this case, since an inkjet head includes many actuators, the increase in the cost of the head drive control device becomes large.
Besides, an actuator in a drop discharge head needs to have a function of bending a diaphragm toward electrodes by turning a voltage on between the electrodes, and a function of returning the diaphragm to the original position by turning the voltage off. Therefore, from a functional viewpoint, there is no need for using a bias method as used in the driving method described in PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998 “A MEMS-Based Projection Display” as above; instead, applying a required potential to one electrode and setting another electrode to GND may be sufficient. Yet, when using the bias method, the voltage does not need to be changed positive and negative upon each control (for discharging one drop); rather, this impairs functions of the head. Besides, it is not necessary to apply an especially large potential to one of electrodes. Therefore, the driving method described in PROCEEDINGS OF THE IEEE, VOL. 86, NO. 8, AUGUST 1998 “A MEMS-Based Projection Display” cannot be simply applied to a drive control device for a head.